1. Field of the Invention
The present invention relates generally to nonlinear distortion compensators in radio transmitters, and more particularly to a distortion compensator of a digital pre-distortion type.
2. Description of the Related Art
In this type of technical field, the digital pre-distortion (DPD) technique has been widely used to compensate for the nonlinear distortion of a transmission signal caused by the power amplifier of a radio transmitter.
FIG. 1 is a block diagram showing part of a radio transmitter with a DPD distortion compensation function. Referring to FIG. 1, the radio transmitter includes a DPD unit 102, a digital-to-analog converter part (D/A) 104, a frequency converter part 106, a high frequency amplifier part 108, a distributor part 110, a frequency converter part 112, and an analog-to-digital converter part (A/D) 114. The DPD unit 102 includes an amplitude and phase correction part 116, a look-up table 118, an error detection part 120, and a delay part (D) 122.
A modulated baseband digital transmission signal is input to the DPD unit 102, where the digital transmission signal is corrected by pre-distortion compensation. Thereafter, the digital transmission signal is converted into an analog transmission signal in the D/A 104. The analog transmission signal is frequency-converted to a radio frequency signal by the frequency converter part 106, and is amplified in the high frequency amplifier part 108 to be transmitted by radio from an antenna part (not graphically represented) via the distributor part 110. On the other hand, the transmission signal is extracted by the distributor part 110 as a feedback signal, and is frequency-converted to a baseband signal in the frequency converter part 112 to be input to the A/D 114. The output of the A/D 114 is input to the DPD unit 102. The high frequency amplifier part 108 and the other analog elements may provide distortion to a signal to be transmitted because of not only their own nonlinear characteristics but also their dependence on the environment of use, such as variations in temperature and over time. The distortion is detected in the error detection part 120 of the DPD unit 102 as the difference or error between the digital feedback signal from the A/D 114 and the digital transmission signal, whose timing has been adjusted by being delayed by the delay part 122 so as to be compared with the feedback signal. In the DPD unit 102, the distortion compensation coefficients of the look-up table 118 are adaptively updated so as to reduce the error detected in the error detection part 120. Then, an appropriate distortion compensation coefficient is selected in accordance with the level of the digital transmission signal, and the digital transmission signal is multiplied by the selected distortion compensation coefficient in the amplitude and phase correction part 116. The distortion compensation coefficient functions to predistort the amplitude and phase of the digital transmission signal so as to offset nonlinear distortion components introduced into the transmission signal by the analog elements. For instance, Japanese Laid-Open Patent Applications No. 63-208330, 2001-345718, and 2001-189685 disclose distortion compensators as described above.
The amplitude of the modulated baseband digital transmission signal varies in level from high to low. Therefore, a feedback signal of a high amplitude level is obtained for a digital transmission signal of a high amplitude level, and a feedback signal of a low amplitude level is obtained for a digital transmission signal of a low amplitude level. In this case, if the amplitude level of the analog feedback signal input to the A/D 114 is lowered, the number of bits effectively used in the A/D 114 is reduced. As a result, the accuracy of the digital feedback signal may be reduced.
The lowest amplitude level of a digital transmission signal may be as low as approximately one several-hundredth of its highest amplitude level, or be lower than its highest amplitude level by tens of dB. If the number of bits of the A/D 114 is 12, the A/D 114 may represent 212=4096 amplitude levels. If the highest amplitude level, which is 4096, is reduced to 1/128 (2−7) thereof, the amplitude level becomes 32=25, which is a reduction of approximately 20 dB. When the amplitude level is 32, only five of the 12 bits of the A/D 114 are used, which means that the remaining seven bits are not effectively used. If the number of bits effectively used in the A/D 114 is thus reduced, the accuracy of the digital feedback signal generated therein and the accuracy of the error signal generated in the error detection part 120 may be decreased, resulting in a reduction of the accuracy of distortion compensation.